The present invention relates to a lens grinding apparatus for grinding a lens such that it conforms to the shape of an eyeglass frame.
Some types of lens grinding apparatus are adapted to have a capability for performing polishing (specular processing) such that an edge surface of the lens is polished to have a mirror-like gloss.
Conventionally, the amount of polishing to be performed after finish processing is typically set to 0.1 mm and the entire periphery of the lens is processed with its edge surface being pressed into contact with a polishing abrasive wheel such that the distance between the abrasive wheel rotating shaft and each of the lens rotating shaft will vary by that amount of polishing.
A problem with this method is that the amount of lens grinding decreases as the point of processing (i.e., the point at which the edge surface of the lens is pressed into contact with the abrasive wheel) departs from the line connecting the abrasive wheel rotating shaft and each of the lens rotating shafts.
Furthermore, if the point of processing does not lie on the line connecting the abrasive wheel rotating shaft and each of the lens rotating shafts, the lens relief that is caused by the rigidity of the lens chucking/holding portion has an effect on the amount of processing such that it decreases even if the processing pressure is the same. Depending on the actual setting of the amount of polishing (specular processing), a certain part of the lens cannot be ground at all and it fails to be polished to have the desired polished surface.
One may think of increasing the setting of the amount of polishing in order to ensure that no part of the lens will remain unpolished despite the lens relief. If the lens to be processed has a thin edge, this approach is effective and the lens can be processed by an amount substantially equal to the desired amount; however, the amount of grinding is prone to decrease if the lens has a thick edge. As a result, the shape of the polished lens is variable with its edge thickness, occasionally making it impossible to produce lenses that fit snugly to the user's eyeglass frame. To avoid this problem, the amount of polishing is preferably as small as possible.